1. Field of the Invention
This invention relates to the formulation of coating compositions with high polymer solids, more particularly to the formulation of a high solids pigmented film-forming coating composition comprising low molecular weight polyester oligomers by incorporating into the coating composition fumed silica co-dispersed with the pigment mill base.
2. Description of The Prior Art
Of particular interest to the present invention are high solids coating compositions based on low molecular weight polyester oligomers. Such compositions contain at least 50% by weight of film-forming constituents, the constituents typically comprising low molecular weight polyester oligomers with controlled hydroxyl functionality and a curing or crosslinking agent for the oligomers such as an aminoplast resin or organic polyisocyanate. These compositions can be applied to a substrate at relatively high solids levels and cured at commercially acceptable temperatures to provide a durable, flexible but hard, crosslinked polymeric finish while maintaining a desirable low level of solvent emissions during the coating process.
Blends of low molecular weight hydroxyterminated polyester based on diols such as 2,2,4-trimethyl-1,3-pentane diol and diacids or their esters, such as isophthalic acid, and incorporated into high solids coating compositions are disclosed in U.S. application Ser. No. 947,183, filed Sept. 29, 1978. Similar disclosure is found in U.K. Pat. No. 1,561,076, granted Apr. 16, 1980, incorporated herein by reference.
High solids coating compositions with film-forming constituents comprising a polyester polyol based on hindered glycols and optionally an epoxy resin or the esterification product of the epoxy resin with a monocarboxylic acid are disclosed in U.S. application Ser. No. 174,255, filed July 31, 1980. Similar disclosure is found in Belgium Pat. No. 887,266, granted July 28, 1981, incorporated herein by reference. Disclosed therein is a film-forming blend comprising a polyester polyol that is the reaction product of
(1) neopentyl glycol and at least one other hindered diol containing two methylol groups wherein each methylol group is attached directly to a cycloaliphatic or aromatic structure or to a tertiary carbon atom, the molar ratio of neopentyl glycol to hindered diol being 2:1 to 6:1, and PA1 (2) a mixture of an aromatic and an aliphatic dicarboxylic acid wherein the molar ratio of aromatic acid to aliphatic acid is from 2:1 to 10:1,
wherein the molar ratio of (1) to (2) is from 1.3:1 to 1.9:1 and wherein the polyol has a hydroxyl content of about 3.0-10.0% by weight;
The use of fumed silica as an additive for coating compositions is known in the art. Silica serves an important role for flow control to reduce sagging of a finished coating composition after application of the composition and during baking or curing of the coated film. U.S. Pat. No. 4,238,387, issued Dec. 9, 1980 to J. A. Antonelli et al. discloses an additive comprising fumed silica for high solids coating compositions to provide pseudoplastic rheology to the compositions.
An important optical property for a formulated coating composition is it's color stability. Colorimeters and spectrophotometers are well-known in the art and are used to measure certain optical properties of various paint films which have been coated over test panels. A typical spectrophotometer provides for the measurement of the amount of light reflected at varying light wavelength in the visible spectrum by a painted panel that is held at a given angle relative to the direction of an incident source of light. The reflectance factor of the coating composition enables paint chemists to calculate color values by which to characterize the color of the coating compositions.
For example, a coated panel is prepared using a particular coating composition and reflectance data is measured for that panel. The coating composition itself may then be aged for a given time and under given conditions of storage. A coated panel is then prepared utilizing the aged composition and reflectance data is measured and compared with that originally measured. Change in data reflects change in color, or "color drift" upon aging. The closer the compared values, the better the color stability for that coating composition.
In the formulation of pigmented high solids coating compositions based on low molecular weight polyester oligomers, certain problems in color stability become apparent. Stability tests on certain blends of colored mill bases with white enamels, i.e., lightly tinted compositions, drifted light upon aging of the composition. Enamel compositions tinted with inorganic pigments such as chrome yellow, molybdate orange, iron oxide red, ferrite yellow, and titanium yellow drifted toward deeper, more saturated colors, while enamels shaded with the organic pigments tended to have normal color stability.
These problems with color stability cannot be tolerated since it is clear that if there is color drift for a coating composition under various conditions of storage, it would be very difficult to match tints and to achieve constant color matches.